In mobile communications, a short-time variation at the reception level called fading (refer to FIG. 18) is known to occur. With recent mobile communications, the technology of adaptive modulation has been widely used for overcoming any degradation of properties in a fading environment. Adaptive modulation is a technique for changing the transmission format in accordance with the reception level at the reception end. Selecting an appropriate transmission format at various points in time will enable communications of a satisfactory level even in the fading environment.
A device at the transmission end performs such adaptive modulation using the quality of reception at the reception end. A device at the reception end sends information about the measured reception quality, i.e., CQI (Channel Quality Indicator), to the transmission end, and the device at the transmission end determines the transmission format based on the information about the sent reception quality (refer to FIG. 19).
However, in a high-speed fading environment, the reception quality varies between the time of measurement and the time of reception. As a result, the adaptive modulation may not work effectively, thereby possibly causing degradation of properties. In FIG. 18, the CQI is measured by an antenna 11 provided on a communications device mounted in a vehicle 1. The resulting CQI is transmitted to a base station 2. In response thereto, the base station 2 returns, to the vehicle 1, data in the transmission format corresponding to the provided CQI. The problem here is that when such data is provided to the vehicle 1, the vehicle 1 is already at a different location. If moving at a high speed, the vehicle 1 moves further before it is provided with the data, and thus the level of the reception quality of the antenna 11 may be different from the level at the time of transmitting the CQI. It is thus highly likely that the transmission format of the data provided by the base station 2 is not suitable for the present reception quality.
For such reasons, when AMC (Adaptive Modulation and Coding) being a part of the adaptive modulation is performed with HSDPA (High Speed Downlink Packet Access), for example, in an environment of 120 km/h (VA 120), the maximum throughput is known to fall about 15% compared to an environment of 30 km/h (VA 30). In the near future, for the purpose of increasing the communications speed, the frequency of radio waves used in communications is expected to be increased, and thus the degradation of properties is likely to be accelerated even more in the high-speed fading environment.